This invention relates generally to machines and procedures for incremental printing or copying of text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a machine and method that constructsxe2x80x94under direct computer controlxe2x80x94text or images from individual colorant spots created on a printing medium, in a two-dimensional pixel array. For purposes of this document, by the phrases xe2x80x9cincremental printingxe2x80x9d and xe2x80x9cincremental printerxe2x80x9d it is meant to encompass all printers and copiers that perform computer-controlled construction of images by small increments.
Incremental printers thereby form images either directly on the print mediumxe2x80x94as in the case of ink-jet, dot-matrix or wax-transfer systemsxe2x80x94or on an electrostatically charged drum just before transfer to the medium as in the case of laser printers. Thus by xe2x80x9cincremental printerxe2x80x9d it is meant to exclude printing presses, which form a whole image from a previously prepared master negative or plate. The invention relates most particularly to hardware for use in calibration to optimize color effects, prevent overinking, and perform other functions directly related to image quality.
1. Introduction
Printer users have a need for accurate color reproduction, for a very great variety of reasons. Many businesses depend on color for their image recognition and identification. Even the optimum maintenance of trademark rights in some situations can depend upon accurate presentation of the color portions of a mark.
Much more familiar motivations include the desire of hobby and home users to see natural flesh tones in printed reproductions of photographs, and to see colors in graphic designs that match their originals.
Colors machine-printed as arrays of ink dots are affected by a wide range of factors including temperature, humidity, ink viscosity, absorption by paper or other printing media, writing-mechanism wear, and many others. All these factors cause variation in inkdrop volume and thereby dot size on the media.
Efforts to analyze such factors and take them into account typically center about optical measurements of one type or another. These may be made at the factory for a complete line of printers, or made in the field for a single production unitxe2x80x94or skilfully devised combinations of these alternatives.
U.S. Pat. No. 5,537,516 of Sherman et al. offers (columns 2 and 3) a brief but helpful orientation as to the differences between measurements respectively made with a densitometer, a colorimeter and a scanner. Sherman also offers several proposals for using a scanner to calibrate a printer.
These proposals include various regimes of combined factory and field measurements, linked through specially constructed standard or customized target test patterns. Sherman also teaches defocusing or diffusing the targets to minimize adverse characteristics of scanners.
Although color accuracy of chromatic colors is of enormous importance commercially, for purposes of the present document (including the claims) the word xe2x80x9ccolorxe2x80x9d is used to encompass both chromatic and nonchromatic colors. Similarly the term xe2x80x9ccolorantxe2x80x9d is used to encompass both chromatic and nonchromatic colorants.
General phrases such as xe2x80x9ccolor measurementxe2x80x9d are used to encompass both densitometry and colorimetry. In particular they encompass measurement of exclusively nonchromatic colors, as well as measurement of chromatic colors either alone or mixed with nonchromatic colors.
U.S. Pat. No. 5,272,518 of Vincent, assigned to the Hewlett Packard Company, describes a small handheld calorimeter for use in calibrating incremental printers and other image-related devices associated with computers. To exclude ambient light the device includes a hood that is meant to be manually brought down directly against a calibration test pattern.
Vincent at one point may seem to suggest too that a calorimeter such as his invention may be incorporated into the printer or other device to facilitate autocalibration; however, Vincent does not teach how to implement any such suggestion. In addition, Vincent teaches extensively the theoretical foundations of calibration for image-related devices of the type under consideration here.
It is known in handheld calorimeters and the like to use a gas-arc flashlamp, particularly for the benefits of the broad, relatively flat and somewhat controllable spectral emission of such a lamp. Neither the Vincent system, however, nor any known system of light measurement used in a printer, employs such a lamp.
2. Densitometry
For a given set of inks with known spectral values and a known printing medium, one can calculate a color table that maps a desired color in some color space into a set of values to be printed on the media. These values may be given as a percentage of the medium to cover with each of the inks.
A color table is created for each unique combination of ink and printing medium. To compensate for dot-size variation, the color table should be adjusted or calibrated for the current operating conditions.
One way to accomplish this is through a density measurement for each of the inks used, by first printing a series of swatches at various nominal (intended) densities, then measuring the actual density of the samples. What is measured is the fraction of the medium that is covered by the dots, and in most densitometer methodologies the actual color does not matter.
This process depends on the composition of the ink remaining constant, and likewise the spectral characteristics of the medium. Typically these tables are computed during development of a printer, and stored permanently in the printerxe2x80x94where they can be changed only by replacing the software storage component, typically a read-only memory (ROM) circuit board.
Through proper use of such measurements, it is possible to compensate for all the factors that affect dot sizexe2x80x94thus making the color output of the printer more consistentxe2x80x94but the calibration is valid only for a current set of environmental conditions, inks and media. A change in temperature therefore would require a new calibration.
Later calibration is not possible with a different medium for which no color table exists. Also it is assumed that the colors do not interactxe2x80x94each ink is linearized independently of the others, in a one-dimensional calibration.
3. Colorimetry
To extend the calibration process to be more general, it is necessary to measure actual spectral values of ink at different levels of coverage on the desired medium. This accounts for interaction of inks and media, and makes the process independent of foreknowledge of ink and medium spectral characteristics.
In this process there is an interaction between the ink colors, because of the overlap between the spectra of the different inks. Although an ink is treated as contributing color in a single spectral band, essentially every ink actually has components in more than one part of the spectrum.
This is a multidimensional calibration. This process creates custom color tables for current ambient conditions and arbitary ink and media. In addition, such measurements in effect linearize the first type of calibration mentioned above.
4. Methods
At least two methodologies are known heretofore for calibration of incremental color printers:
(a) Off-line calibrationxe2x80x94In this approach a user operates a spectrally discriminating optical sensing device, i. e. a calorimeter, to make measurements of a test pattern. The calorimeter readings are taken independently of the printer operation.
First the printer must be used to print the test pattern onto the desired medium. Modernly this process is controlled by an application program in a host computer or in an onboard microprocessor that is part of the printer itself. The pattern usually includes many color patches, typically between fifty and several hundred.
Then the user must operate a calorimeterxe2x80x94such as for example a small unit sometimes called a xe2x80x9ccolor mousexe2x80x9d. (The term xe2x80x9ccolor mousexe2x80x9d appears to be related to, but not one of, the trademarks of the Color Savvy Company.)
Alternatively the user may use a spectrophotometer. In either case, the equipment is used to measure the patches one by one while the readings are processed by the application program. The application in turn creates a custom color table for the instant set of conditions.
The application then can send accurate color values to the printer (which should not modify them). If the temperature or another condition changes, then the calibration should be done again.
Problems with this method include the amount of time required of the user to carry out a tedious process, and the likelihood of error. For example, the user may place the sensor over a patch other than the one expected by the system.
Data obtained are ordinarily exterior to the printer and require use of an external processor, though the data may be downloaded to the printer if the system is so configured. (Another issue in some parts of the world is the physical space required to put down a print sample with swatches on a level surface for measurement.)
(b) Automatic on-line calibrationxe2x80x94A second method is automatic and was pioneered by the Hewlett Packard Company in its DesignJet(copyright) 2500CP printer. That product uses a sensing element designed for other purposes (determining pen alignment and pen condition) to make a rough density measurement.
Examples of such sensing elements and their uses appear in U.S. Pat. No. 5,600,350 of Cobbs et al. (assigned to the Hewlett Packard Company) as well as the copending patent documents listed earlier. In general these sensing elements are very rough in comparison with true densitometers, but very slightly modified to provide some selective spectral sensitivity to the several inks involved.
As suggested by Cobbs, a representative low or lowest printing speed is e. g. roughly 13 inches per second (ips), or about 34 cm/sec. Cobbs likewise indicates that a representative intermediate speed is roughly 17 ips (42 cm/sec) and a representative high or highest speed is e. g. roughly 27 ips (68 cm/sec).
In a scanning inkjet printer such as the 2500CP, the sensor is mounted on the moving carriage that holds the inkjet pens. As is well known, the carriage moves the pens back and forth across the printing medium to eject swaths of ink droplets onto the medium, while these swaths are arrayed along the length of the medium by lengthwise advance of the medium, to form the image.
Accordingly, placement of the optical sensing element on the carriage gives the sensor access to essentially the same full area of the printing medium as the pens have. Thus the pens can be used to print test-pattern swatches on the medium, and then after the ink is thoroughly dry the medium bearing the test pattern can be fed through the machine again for measurement.
When applied to color calibration, the sensing element is used to make measurements of swatches that go from white (bare media) to opaque (complete ink coverage), in for example eight steps. Light-emitting diodes (LEDs) are used to illuminate the swatches, while a photodetector reads the amount of light reflected from the swatches. The LEDs are chosen so that the inks absorb the light well, or in other words appear dark to the photodetector.
The detector is moved across the swatches with LED illuminators operating, and the detector readings are recorded. The relative density of each swatch is calculated and used to correct what may be called the xe2x80x9cgainxe2x80x9d of each ink.
Two LEDs are-usedxe2x80x94a green one for use with cyan, magenta and black inks, and a blue one for use with yellow. This method provides a measure of feedback to keep the color of a printer relatively constant, but does not provide an absolute color specification. It requires lookup tables prepared in advance for each combination of ink and printing medium.
This method, even with its simple brightness measurements combined with selective spectral excitation, still remains something less than densitometryxe2x80x94in this document for ease of reference it will be called xe2x80x9cpseudodensitometryxe2x80x9d. The use of a blue LED for detecting the yellow ink is adopted merely as a means of being able to detect that color of ink with anything approaching adequate signal-to-noise ratio.
Thus pseudodensitometry does not at all closely approach colorimetry. Problems with this method include these:
1) As the detector moves, it cannot touch the medium and so is held about 1.5 mm above the medium. This standoff spacing allows ambient light to enter the detector where it generates noise and makes readings uncertain.
2) Ink-aerosol particles from the printing process drift through the atmosphere above the medium and onto optical surfaces and coat those surfaces. There are two adverse effects: (a) the coating reduces the amount of light transmitted, making the measurement less sensitive, and (b) as the particles are colored they selectively distort the light which they pass through or reflect.
A fixed cover glass is used to protect the optical elements from aerosolxe2x80x94and when light transmission falls below acceptable levels, the user is prompted to replace the glass. In the meantime the system suffers the progressively drifting color inaccuracy just described at (b).
Historically the required replacement frequency has been about once a year. Recent data, however, suggest that somewhat more-frequent replacement is in order. With a true calorimetric system, replacement would be required significantly more often.
3) No absolute reference is used except the bare medium.
4) No colorimetric data are possiblexe2x80x94only density.
5) The full-ink-coverage point is not accurate. The printer can only print one dot at each addressable location, and in the worst case these dots do not completely cover the medium. Therefore the nominal-full-coverage point is not really measured with full coverage, but the software has to assume that it is.
6) Color tables are available for only a few media. Arbitrary media must be operated on a completely open-loop basis.
7) Variation in sensor-to-medium distance changes the calibration.
5. Conclusion
As shown above, problems of color consistencyxe2x80x94and calibration such as needed to achieve itxe2x80x94have continued to impede achievement of uniformly excellent inkjet printing on various industrially important printing media. Neither the time-consuming and error-prone colorimetric method, on the one hand, nor the automated but fundamentally inaccurate pseudodensitometric method, on the other hand, is able to provide fast, reliable, high-quality but economical performance.
Precisely that kind of performance is essential in the highly competitive field of modern incremental printing. Thus important aspects of the technology used in the field of the invention, particularly with regard to hardware systems for use in efficient and accurate calibration of printers, remain amenable to useful refinement.
The present invention introduces such refinement. Before offering a relatively rigorous introduction to the invention, this text will provide some informal comments that may be helpful in orientation. These remarks have been reserved for the present section of this document because they are in no way a part of the prior art (or parallel developments) in color calibration. It is to be understood that these preliminary comments are not a definition or description of the invention.
As suggested in the preceding xe2x80x9cBackgroundxe2x80x9d section, the theory and procedures of calibration have been well-elaborated in the art, but available hardware heretofore has not been adequate. For an inkjet printer, a first step according to the present invention is to consider installing into the printer a calorimeter, rather than basically a pseudodensitometer as in method (b) above.
Vincent may suggest something of the sort; however, like the pseudodensitometer the colorimeter must be moved around to measure swatches. One question is how to accomplish that.
A natural start according to the present invention is simply to mount a calorimeter such as Vincent""s directly on the scanning pen carriage, as done for the pseudodensitometer. An obstacle arises immediately as commercially available colorimetersxe2x80x94even the xe2x80x9ccolor mousexe2x80x9d devicesxe2x80x94are far too bulky and heavy to be so mounted.
The Vincent type is greatly advanced in comparison with earlier devices described in Vincent""s introduction. Nevertheless it is plainly not designed or suitable for either installation or operation on a pen carriage.
A colorimeter typically requires some provision for spectral selection that is better coordinated with the sensitivities of the human eye than the simple ink-related LED colors of the pseudodensitometer. The calorimeter accordingly may have rotating filter wheels or other mechanically elaborate components that would be impractical to operate on a scanning inkjet-pen carriage.
In this regard it is necessary to appreciate some limitations of the scanning carriage. The carriage is part of a multifaceted printing system that is extremely well optimized for the highest possible image quality and the highest possible throughput.
No part of that system can be significantly perturbed without disturbing this delicate balance of electronics, mechanics, thermodynamics, fluid dynamics, chemistry, and economics. In particular the carriage must be accelerated to printing speed and decelerated to a stop for each pass of the printing elements across the medium.
The acceleration and deceleration demands naturally limit the maximum mass that the carriage can bear, to ensure a proper lifetime for the components of the carriage movement system. Assuming that the drive motor can deliver adequate torque to accelerate and decelerate the carriage to and from printing speed within the necessary times and distances, a more massive carriage or components on the carriage introduce more heat, stress and wear and thus a shorter life for the whole system.
In addition the dimensional envelope of the carriage assembly is restricted by the presence of ink containers, user access for replacement, replenishment or servicing of those containers, drive electronics, connecting drive cables, and a position-encoding strip that must be threaded entirely through the carriage. For all these reasons a color sensor even remotely the size or mass of Vincent""s, for example, would be wholly impractical to mount on a conventional inkjet printer carriage.
It will be understood that design of a colorimeter small and lightweight enough to be suitable for such mounting is a major project in itself, and relatively daunting. The heart of such a new calorimeter is one principal thrust of the present document, but some innovations introduced in this document instead pursue an alternative approach without a new lightweight colorimeter.
One consideration that can be exploited to provide such an alternative solution to the calorimeter problem is that color calibration is performed very infrequently, in comparison with the conventional movements of an ink-jet pen carriage. One estimate is one color calibration for each 10,000 to 30,000 printing passes.
This consideration suggests that placing the color sensor on the carriage would add weight, bulk, stress, wear and complexity which would be rarely usedxe2x80x94and therefore extremely cost-inefficient. Implementing a color sensor in a different location would therefore be more advantageous.
Still, the carriage is appealing because it provides access to all the necessary parts of a test pattern and already has the necessary associated components for both motive forces and positional determination. The sensor must be moved to each of the test-pattern patches (or the patches to the sensor, or some of each).
One other type of printer subsystem has a comparably very low duty cyclexe2x80x94namely a paper-cutter wheel that is used to slice off completed drawings from a continuous roll of printing medium. It is known to operate such a paper cutter on a separate carriage that need not be accelerated and decelerated dozens of times per image.
The separate carriage in that case is not provided with its own drive cables or position-determining components, but rather is coupled to the main carriagexe2x80x94for positioning by those components already associated with the main carriage. No such auxiliary carriage, however, has ever been used for positioning a module or subsystem that is directly related to color calibration, color refinement, or indeed any other aspect of image quality.
With these introductory comments in mind, this document will now continue with a more-formal presentation of certain aspects of the invention.
In its preferred embodiments, the present invention has several aspects or facets that can be used independently. With limited exceptions that will shortly become clear, the several facets are preferably employed together to optimize their benefits.
In preferred embodiments of a first of its facets or aspects, the invention is an incremental printer for forming desired images on a printing medium, by construction from individual marks in arrays. The printer includes at least one colorant-placing module for marking on the medium.
It also includes a first sensor for determining condition or relative positioning (or both) of the at least one colorant-placing module; and in addition a second sensor for making color measurements of marking arrays formed on the medium by the at least one module.
In this document (including the claims), as noted earlier the term xe2x80x9ccolorantxe2x80x9d encompasses nonchromatic colorant; and the phrase xe2x80x9ccolor measurementsxe2x80x9d encompasses both densitometry and colorimetry. The phase xe2x80x9crelative positioningxe2x80x9d encompasses positioning of a single colorant-placing module relative to its carriage or the printing system generally, and also encompasses positioning of plural colorant-placing modules relative to one another. As will be clear, the first sensor may take the form of separate sensors for determining condition and positioning respectively.
The foregoing may constitute a description or definition of the first facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention significantly mitigates the difficulties left unresolved in the art.
In particular, the invention provides a color-calibration sensor that is distinct and separate from the carriage-mounted sensor used for pen alignment, detection of empty ink cartridges or inkdrop size, or identification of malfunctioning nozzles. As a result the designer of a printer is enabled to decouple the color-calibration system design from the limitations of the carriage-mounted pen alignment/status sensor.
In other words, it becomes possible to solve the special problems of color calibration without insisting upon compatibility of the two disparate sensing functions. Detailed results of such less-restricted design will be seen later in this documentxe2x80x94but those further inventive details in a certain sense flow from the innovation of this first aspect of the invention.
Although this aspect of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits. For example preferably the second sensor is for making calorimetric measurements.
It is also preferred that the printer additionally include a colorant carriagexe2x80x94for scanning the at least one colorant-placing module over the printing medium. Also preferably the first sensor is mounted to the colorant carriage but the second sensor instead is mounted independently of the first sensor.
In this case it is further preferred that the printer also include an auxiliary carriage for holding the second sensor and scanning the second sensor over such medium. This auxiliary carriage in turn preferably is selectively attachable to and detachable from the colorant carriage.
Another basic preference as to the first aspect of the invention, in certain embodiments, is that the printer include some means for excluding ambient light from the second sensor during the making of color measurements. For purposes of generality and breadth in discussion of the invention, in the present document these means will be called simply the xe2x80x9cambient-light excluding meansxe2x80x9d.
Preferably these ambient-light excluding means include a hood generally surrounding the second sensor laterally with respect to a sensing direction, and a mechanism for advancing the hood along the sensing direction toward the medium.
Still other preferences as to the first facet of the invention, in certain embodiments, are that the printer include a mechanism for advancing the second sensor into a measurement positionxe2x80x94and a mechanism for advancing the second sensor into contact with the medium. In addition preferably the printer includes means for presenting at least one color reference target to the second sensor. Again for generality and breadth these means will be called, in this document, the xe2x80x9cpresenting meansxe2x80x9d.
In preferred embodiments of a second of its main aspects, the invention is an incremental printer for forming desired images on a printing medium, by construction from individual marks in arrays. The printer includes at least one colorant-placing module for marking on the medium.
It also includes a first carriage for scanning the colorant-placing module over the medium. In addition it includes a second carriage, discrete from the first carriage, for use in refining the quality of images produced by the printer.
The foregoing may serve as a description or definition of the second facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention too significantly mitigates the difficulties left unresolved in the art.
In particular, in this facet of the invention the source of certain previously discussed limitations of the prior art is now localized in the scanning carriage. This is a major conceptual step from the summary of the preceding xe2x80x9cBackgroundxe2x80x9d section of this documentxe2x80x94which could only point in a much more abstract way to xe2x80x9ctime-consuming and error-pronexe2x80x9d colorimetry and xe2x80x9cautomated but fundamentally inaccuratexe2x80x9d pseudodensitometry.
As seen in the light of this second aspect of the invention, what makes colorimetry or true densitometry time consuming and error prone is its historical inaccessibility to the already-available carriage (due to overly bulky or heavy components used in colorimetry). What makes pseudodensitometry fundamentally inaccurate is that it is limited to what can be carried on the already-available carriage.
The second facet of the invention, now under discussion, makes it possible to break out of this circular-seeming conundrum. This is accomplished by providing two separate and distinct carriagesxe2x80x94once again to decouple the requirements of color measurement from those of the printing process itself, and from those of relatively primitive pen-status or alignment systems.
Although this facet of the invention in its broad form thus represents a significant advance in the art, it is preferably practiced in conjunction with certain other features or characteristics that further enhance enjoyment of overall benefits. For example preferably the second carriage is selectively attachable to and detachable from the first carriage.
Also it is preferable that the second carriage scan a sensor over the medium. In this case, still more preferably the sensor is a sensor for making color measurements of marks formed on the medium by the at least one colorant-placing modulexe2x80x94and preferably the second carriage also holds at least one reference target for presentation to the sensor. (Alternative mounting of targets stationarily, to fixed components of the printer, will be taken up shortly.)
As to the last-mentioned preference, the second carriage itself actually holds not only the sensor but also a target for the sensor to view. This target may be made to function as an absolute calibration standardxe2x80x94which enables the system to escape from a previously discussed handicap of automatic in-printer calibration, namely the absence of an absolute standard. In this regard preferably the sensor is a calorimetric sensor, and the reference target is a calorimetric reference target.
Yet another preference is that the printer also include a hood generally surrounding the sensor laterally with respect to a sensing directionxe2x80x94and a mechanism for advancing the hood along the sensing direction toward the medium. It is also preferable that the printer include a mechanism for advancing a component associated with the sensor into contact with the medium.
Such a component, merely by way of example, might be the hood or a compliant facing fixed to the hood. In addition this second facet of the invention is amenable to other applicationsxe2x80x94as for instance a video camera or the like mounted to the second carriage can usefully measure image-quality-related parameters other than color.
In preferred embodiments of a third basic aspect or facet, the invention is an incremental printer for forming desired images on a printing medium, by construction from individual marks in arrays. The printer includes at least one colorant-placing module for marking on the medium, and a sensor for measuring color properties of colorant marked on such medium by the colorant-placing module.
In addition the printer includes a hood for excluding ambient light from the sensor during the color-property measuring. The hood generally surrounds the sensor laterally with respect to a sensing direction. In addition the printer has a mechanism for automatically advancing the hood along the sensing direction toward the medium.
The foregoing may constitute a description or definition of preferred embodiments of the third facet of the invention in its broadest or most general form. Even in this general form, however, it can be seen that this aspect of the invention significantly mitigates difficulties left unresolved in the art.
In particular, the mechanism described is able to minimize the admission of ambient light into the color-measuring systemxe2x80x94and to do so more effectively than is possible by carrying an ambient-excluding hood always at the same distance needed for effective clearance during movement of the sensor into position.
Nevertheless, as before, for maximum enjoyment of the benefits of the invention preferably certain additional features or characteristics are included. For instance, it is preferable that the hood-advancing mechanism also automatically advance the color sensor into a measurement position.
Also preferably the hood includes, at a forward surface, a compliant material for facilitating an effective contact between the hood and the printing medium. Another preference is that the hood be movable with respect to the sensor; and that the mechanism advance the hood with respect to the sensor. For best exclusion of ambient light, the hood (or its compliant facing) is advanced into contact with the medium.
Another preference is that the printer include a door for protecting the sensor when not in use, and that the hood-advancing mechanism also include some means for opening the door for measurements by the sensor. Other preferences as to the door will appear shortly.
In preferred embodiments of a fourth of its aspects, the invention is an incremental printing system for forming desired images on a printing medium. The printing system forms the images by construction from very large numbers of individual liquid-ink drops ejected onto such medium in arrays. (Typical images modernly include many thousands of drops per square centimeter.)
The printing system includes at least one colorant-placing module for ejecting very large numbers of liquid-ink drops onto the medium. This ejection occurs substantially whenever the printing system is in use for forming images.
Also included in the printing system is a sensor, having at least one optical surface, for infrequently measuring color properties of ink previously received on the medium from the at least one colorant-placing module. This measuring occurs substantially only when the printing system is not in use for forming images.
The printing system further includes an automatic microprocessor for using the measured color properties in refining operation of the colorant-placing module. The printing system uses these measured properties to optimize the quality of images formed on the medium thereafter.
In addition the printing system includes a door for protecting the at least one optical surface of the sensor from being coated by atmospherically carried residual liquid ink. This protection is provided when the sensor is not in usexe2x80x94particularly including whenever the printing system is in use for forming images.
Yet additionally included is a mechanism for automatically opening the door before use of the sensor, and for automatically closing the door after use of the sensor. This mechanism enables the microprocessor to reliably optimize the quality of images, free from degradation of the measured color properties by coating of liquid ink on the at least one optical surface.
The foregoing may describe or define preferred embodiments of the fourth facet of the invention in its broadest or most general form. As will be understood, in this printing system the microprocessor may be the general-purpose processor in an associated computer, or can be a programmed microprocessor in a printer product. (By that is meant a printer case that includes the sensor, the colorant-placing module or modules, whatever mechanisms discharge those modules and position them with respect to the printing medium, and associated componentry).
If in the printer, the processor can take the form of a general-purpose processor holding a program, or reading program modules from an associated read-only memory (ROM); or the processor may be an application-specific integrated circuit (ASIC). Alternatively still, the processor can be in another separate enclosure, e. g. a raster image processor (RIP). Such RIP devices are available nowadays for use with computer-controlled printers, to avoid tying up either the computer or the printer.
This fourth aspect of the invention addresses and resolves the problems of the contaminated cover glass discussed earlier in the xe2x80x9cBackgroundxe2x80x9d section. As will be seen this facet of the invention can also be exploited in connection with the lack of an absolute standard in some color-measurement systems.
This aspect of the invention is preferably practiced in conjunction with optimizing characteristics. For example preferably the door-opening-and-closing mechanism automatically opens the door substantially in preparation for use of the sensor; and also automatically closes the door promptly after use of the sensor. In some embodiments the door-opening mechanism moves the sensor into a measurement position as well.
If the sensor has multiple optical surfaces, preferably the door protects all of them from being coated with ink. Some embodiments may have two or more sensorsxe2x80x94e. g., a sensor for measuring color properties of the previously received ink; and a separate sensor for determining, from patterns of the previously received ink, condition of the at least one inkdrop-placing module.
Such condition may include whether the module is out of ink. If there are plural placing modules, the separate sensor may be for use in determining, from patterns of the previously received ink, either the condition just described, or relative positioning of the inkdrop-placing modulesxe2x80x94or both. This fourth facet of the invention, however, is also applicable to printing systems in which a single sensor is used for color measurement as well as the condition or positioning determinations just discussed.
Also preferably this aspect of the invention includes some means for measuring at least one absolute color reference, when the door is not open. (By xe2x80x9cnot openxe2x80x9d is meant that the door is not admitting color characteristics of the previously received ink to the sensor.) For generality and breadth these means will be called the xe2x80x9cabsolute-reference measuring meansxe2x80x9d.
In this case it is further preferable that the absolute-reference measuring means include at least one color reference target that is exposed to the sensor when the door is closed. When such a target is included, it is preferably carried on a surface of the door.
Another preference is that the door take the form of a shutter. In this case it is preferable that the shutter be in a plane generally parallel to the printing medium, and that the shutter slide open and shut generally within that plane.
A fifth facet or aspect of the invention is, in its preferred embodiments, an incremental printer for forming desired images on a printing medium, by construction from individual marks in arrays. The printer includes at least one colorant-placing module for marking on the medium, and a sensor for measuring color properties of colorant marked on the medium by the colorant-placing module.
Also included is a flashlamp for illuminating colorant marked on the medium at an intensity high enough to make ambient light substantially insignificant to the measuring process.
The foregoing may be a broad, general definition or description of the fifth aspect of the invention. As will be understood, this facet of the invention is particularly valuable for its virtually complete elimination of any need to shield the sensor from ambient light.
From the familiar use of flashlamps in photography it is well known that such lamps are readily made bright enough to essentially swamp out normal room illumination and in many cases even moderate daylight. (This is not to say that the sensor of this fifth facet of the invention is necessarily intended for operation outdoors in direct sunlight; the sensor can function well within a generally conventional printer cabinet, with the usual minimal shielding.)
Thus according to this aspect of the invention the sensor requires no large hood, and no mechanism for advancing the sensor into or away from contact with the print medium or the ink thereon. In fact the sensor requires no mechanism for advancing the sensor along the measurement direction at all.
Previous colorimeters using flashlampsxe2x80x94essentially for the benefit of their spectral distribution, as mentioned earlierxe2x80x94have employed hoods and in general have required manual advance of the hood along the measurement direction and into contact with the medium bearing the printed test pattern.
According to this facet of the invention in comparison, a great simplification is effected, and with relatively little handicap in terms of weight, bulk, or cost. Some electronic complexity is added.
As this facet of the invention has minimal need for shielding of the sensor against ambient light, preferred characteristics and features for this facet of the invention in fact include minimal provision of such shielding. Weight, bulk and cost benefits are thereby enhanced.
It is also preferable that, during the measuring, the sensor is in contact with neither the medium nor colorant marked on the medium. Mechanical simplification is thereby optimizedxe2x80x94and because of the brightness and resulting virtually complete elimination of ambient shielding, the sensor is made and operated very differently from previous, handheld calorimeters fitted with flashlamps.
Another preference is that the flashlamp in fact operate in a flashing mode. In particular the lamp is best flashed for a time interval short enough to make energy consumption and heating by the flashlamp substantially insignificant.
A preferred embodiment of the invention in yet a sixth of its major facets or aspects is an incremental printer for forming desired images on a printing medium. The printer does so by construction from individual marks in arrays.
The printer includes at least one colorant-placing module for marking on such medium; and a sensor for measuring color properties of colorant marked on such medium by the colorant-placing module. In addition the printer includes a moving carriage for automatically positioning the sensor over colorant on such medium.
Further included is at least one reference target disposed for exposure to the sensor to provide a colorimetric reference measurement. This measurement is for use in conjunction with the measured color properties of colorant marked on the medium.
The foregoing may represent a description or definition of the sixth independent aspect or facet of the invention in its most general or broad form. Even in this form, however, it can be seen that this sixth facet of the invention importantly resolves troublesome difficulties of the art.
In particular, an absolute reference measurement can be obtained without going beyond the resources built into the printer. This expansion of resources enables automatic operation of the reference measurement as well as the color-patch measurements discussed earlier.
Although the sixth facet of the invention as couched in its most general form thus importantly advances the art, it is nonetheless preferred to practice this aspect of the invention in conjunction with other features or characteristics that optimize the enjoyment of its benefits. For example, in one preferred form of this sixth facet of the invention preferably the at least one reference target is carried on the moving carriage.
In another preferred form, it is preferred that the at least one reference target be stationary, and the moving carriage comprise means for automatically positioning the sensor over the at least one reference target. In this case it is further preferred that the printer also include a shutter for protecting the at least one reference-target, and some means actuated by the moving carriage for controlling the shutter.
In any event preferably the at least one target includes a white target. Also preferably the at least one target includes a black target. It is preferable too that the at least one reference target include one or more gray targets. Another preference is that the at least one reference target include a chromatically colored target.
The basis for these colorant preferences is well-established, for example in the Bockman and Borrell patent documents mentioned earlier. As those documents show, one of the most difficult colorimetric alignments for an incremental printer is producing accurate grays, and in particular gray-scale ramps; thus the nonchromatic references mentioned above are particularly useful.
Almost as demanding as this type of calibration, however, is the need for accurate presentation of fully saturated primary colorsxe2x80x94and close behind that consideration is the accurate presentation of fully saturated secondaries. In incremental printing, primary chromatic inks are usually cyan, magenta and yellowxe2x80x94crosscombinations of which are used to form the colors usually regarded as primaries, namely red, green and blue (considered secondary inks, for purposes of incremental printing).
Hence red, green and blue targets for comparison are also very useful. When the system has difficulty approximating these as it should, a reason may be that the inks loaded into the system pens are faulty or at least in some way nonstandard, and this condition can be investigated automatically if the system has accurate reference targets for those colors as well.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: